1. Field of the Invention
The present invention relates to a method for producing a semiconductor substrate, utilizing a bonding technique, used for a semiconductor wafer.
2. Description of the Related Art
For example, as a method for bonding two silicon wafers used when producing a silicon substrate having a multilayer structure, there is the following known method, i.e., after the surface treatment was performed to two mirror-polished silicon wafers (i.e., mirror-surface wafers), in a state that two silicon wafers are closely joined, heat treatment is performed to two silicon wafers at high temperature in order to strengthen the joint, and, as a result, two silicon wafers are forcedly bonded to one another.
There are problems, however, in this method as explained below, that is, when a thin film consisting of different material provided by a patterning process is formed on the joined surface of the silicon wafer, or when trench patterns provided by an etching process are formed on the surface of the silicon wafer, the bonding becomes insufficient due to the unevenness (i.e., difference in grade) on the surface at the boundary of the join.
In order to solve the above problems, there is the following known method, i.e., after the deposition of a poly-crystal silicon on the bonding surface of the silicon wafer, the unevenness of the surface of the silicon wafer is covered by the poly-crystal silicon having a flat surface (i.e., mirror surface) obtained by polishing the surface flat and, the bonding process is performed in a state which is covered by the poly-crystal silicon.
In general, the process for depositing the poly-crystal silicon is performed at high temperature (usually, around 1200.degree. C.) by utilizing the normal pressure CVD (Chemical Vapor Deposition) method. In this case, since the thick poly-crystal silicon can be deposited at high speed, there is an advantage that it is possible to ensure a large cutting portion in the polishing process (in some cases, a grinding process may be provided before the polishing process). Further, there is another advantage that it is possible to improve the throughput in the depositing process.
There are problems, however, in the above method as explained below. That is, when the unevenness of the silicon wafer is formed by the trench pattern having narrow and deep shape, i.e., having a high aspect ratio, it is very difficult to completely bury the trench pattern by using the normal pressure CVD method.
On the other hand, there is a pressure reducing CVD method as one example of a depositing method which can completely bury a trench having high aspect ratio (i.e., deep trench) by using poly-crystal silicon. As a film-making conditions, when employing the pressure reducing CVD method, it is possible to obtain a good burying characteristics in conditions which suppressed growth speed, i.e., at low pressure and low temperature (e.g., 650.degree. C. or less).
That is, when the film is made at the conditions of normal pressure, high temperature and high speed, the upper portion of the trench is closed by the poly-crystal silicon at an initial step of the filmmaking so that a cavity may occur in an inner portion of the trench. However, according to the conditions which suppressed the growth speed, there is no cavity in the inner portion of the trench.
When bondihg the silicon wafer for supporting another silicon wafer of the poly-crystal silicon processed by the film-making process and the grinding process, a hydrophilic process is performed to the bonded surface. In the hydrophilic process, for example, each silicon wafer is dipped into the mixed solution of the sulfuric acid and hydrogen peroxide which are maintained to a predetermined temperature, and an oxide film having a thickness of about several nanometers is formed on the surface of the silicon (i.e., the poly-crystal silicon) in order to obtain the hydrophilic property.
Further, after the above processes, the silicon wafer is dipped into super-pure water in order to obtain the hydrogen binding from the hydroxyl group and the water molecule, which are absorbed on the oxide film, so that two silicon wafers are closely joined one another. Finally, the heat treatment is performed to the joined portion of two silicon wafers so that it is possible to obtain the strong binding (i.e., covalent binding).
When the heat treatment in the bonding process is performed at high temperature (e.g., 1000.degree. C. or more), it is possible to obtain a good joined state without a void (i.e., non-joined area). When the heat treatment is performed at the low temperature (e.g., 800.degree. C. or less), there are many joint failures in which the silicon wafers, which are closely joined to one another, break away and many large voids occur although the join is maintained. It is assumed that, as the reason of the above, the gas (for example, aqueous vapour) which occurred on the bonding surface of the silicon wafer is not diffused in the heat treatment at low temperature.
Accordingly, from the above viewpoint, in order to realize the good bonding state between two silicon wafers, it is considered that the high temperature at the heat treatment is preferable when the bonding process is performed. However, when the heat treatment at the high temperature is performed for the bonding process, there are some join failures. Therefore, the inventors of the present invention had some experiments and considerations in order to clarify the reasons of join failure as explained in detail hereinafter.